1. Field of the Invention
The present invention relates to a mounting assembly for an unducted prop engine, and a method relating to the same, and more particularly to such a prop engine having a pusher configuration, with a mounting strut being positioned forwardly of the propeller path, and with pressurized air being blown from a trailing edge of the strut in a manner to alleviate discontinuity in the velocity gradient caused by the wake of the strut to obtain benefits relative to vibration, noise, and performance.
2. Background Art
It is common to mount an engine to aircraft structure by means of an aerodynamically contoured mounting strut interconnecting the engine to the aircraft structure. When the engine is an unducted prop engine of the pusher type, generally the strut will be positioned upstream of the propeller, so that the airstream passing on the two side surfaces of the strut flows through the propeller path. The presence of the strut will generally create a wake which results in a discontinuity in the adjacent airflow, which in turn results in the blades of the propeller experiencing a difference in angle of attack as the blades pass through the wake of the strut. This variation in angle of attack results in a variation in aerodynamic lift loading on the propeller which in turn results in unwanted vibrations being generated, with these vibrations being transmitted through the structure of the engine and through the strut to the aircraft structure. Another consideration is that this discontinuity of airflow through the propeller path generates unwanted noise which can propagate both to the interior of the aircraft as well as to exterior locations.
A further consideration is the effect of boundary layer air adjacent to the juncture location of the strut to the aircraft structure. For example, with the engines being mounted to the aft portion of the fuselage, there is a tendency for a relatively thick boundary layer to develop at the juncture location of the strut with the fuselage. If the propeller is mounted so that the tip path is relatively close to the fuselage, this thicker boundary layer results in a further discontinuity in the airflow passing through the propeller path and aggravates the problems noted above.
The design of engine mounting struts is generally something of a compromise between the aerodynamic and structural requirements. The aerodynamicist would prefer that the strut be as thin as possible to reduce aerodynamic drag, while the structures specialist would prefer to have the strut with a greater thickness dimension so that the engine could be properly supported with structural components with less mass. Further, the aerodynamicist would prefer the chordwise dimension of the strut to be as small as possible to minimize the wetted area. With regard to optimizing performance of the propeller, it would of course be desirable to have the strut totally "invisible" so that the propeller would encounter an airstream which is as uniform as possible.
A search of the patent literature has revealed a number of patents which deal with the interaction of the stationary vanes and rotor blades of the compressor/fan section of a jet engine. These generally have addressed the problems of community noise where engine noise is propagated to exterior locations, such as populated areas adjacent to the airport. The rotor of a fan jet engine may have as many as forty blades, and these rotate at velocities as high as 4,800 revolutions per minute. The noise which is generated is generally at a relatively high frequency (e.g., 3,200 Hz), which generally can be effectively treated by acoustic linings in the nacelles.
For example, U.S. Pat. No. 3,572,960 (McBride) discloses a system where the inlet guide vanes and the rotor blades of the fan of a fan jet engine are provided with slots running lengthwise of the radially outward portion of the trailing edges. Pressurized air is discharged from these slots to minimize the circumferential velocity gradients immediately downstream of the vanes and blades. This is done to reduce the sound generated by the air entering the rotor blades and the outlet guide vanes. To the best knowledge of the applicants herein, while it has been recognized that this system can reduce engine noise, it has not been incorporated in commercial aircraft.
Other patents noted in the search of the patent literature are the following.
U.S. Pat. No. 1,585,218 (Watanabe) discloses what is called, "Soundless Double Propeller for Airplanes". There is a forward and an aft propeller mounted to a shaft, and these propellers are spaced so that with a given velocity of the airplane, the sound waves generated by the two propellers interfere with each other, and it is alleged that these will cancel each other in a manner to reduce the noise that is generated.
U.S. Pat. No. 2,944,729 (Foley et al) shows an axial flow compressor, of the type used in an aircraft, where the stator blades have at the trailing ege of the stator blades slots 22 at the concave surface of these blades near the rear end thereof. The slots 22 are arranged to blow air in a direction normal to the surface of the vane. This is done to modify the airflow in a manner to change the angle of attack of the blades of the rotor stage immediately behind. Also, such slots can be provided in forward and rear sets of vanes so that air inducted into one set of slots can be directed to the interior of the other set of vanes to be discharged through the slots at that other set of vanes.
U.S. Pat. No. 3,420,502 (Howald) shows a fluid cooled airfoil, where cooling air is circulted through the interior of the airfoil and then discharged through the trailing edge. This is intended for use in turbines or the like where the airfoil blades are often exposed to relatively high temperatures.
U.S. Pat. No. 3,934,410 (Williams) discloses a propeller configuration where there is a shroud surrounding the propeller blades. The propeller blades employ a tangential slot along the trailing edge of each propeller blade and air is blown from the tangential slot over the trailing edge. It is stated that the lift is proportional to the momentum flow of the air out of the slot, and is substantially independent of the rotational velocity. This is done to reduce noise and also increase efficiency.
U.S. Pat. No. 4,156,344 (Cuthbertson et al) discloses a system for use in the inlet of the fan of a fan jet engine, where there is provided in the outer duct wall openings to divert a portion of the airflow at a location immediately downstream of the inlet guide vanes. This has the effect of reducing the noise generated in that area.
U.S. Pat. No. 4,255,083 (Andre et al) discloses a system for reducing the noise in the compressor section of a turbine. Air is injected through orifices located in the stationary components in the compressor section. The flow through the orifices is modulated at the frequency of the noise to be reduced and with a phase difference which increases in an incremental manner from one orifice to the next. The acoustic waves generated are opposite in phase with respect to the noise which is to be reduced. This system is also disclosed in U.S. Pat. No. 4,419,045, which is a divisional application of U.S. Pat. No. 4,255,083.
Czechoslovakian Pat. No. 111,822 discloses what appears to be the inlet of a turbine engine, with air being discharged through the trailing edge of the forward set of stator vanes of the compressor section. The translation of that patent is not presently available.
Swiss Pat. No. 225,231 was also noted in the search. A translation of this patent is not presently available, and it is difficult to discern from the drawing the operation of the apparatus shown.